The present invention relates to a method for separating elastic waves that are received and recorded after they have propagated through a medium and, more particularly, for processing those signals recorded during an acquisition carried out by elastic-wave shots from several positions of one or more sources that are arranged at the surface of the medium. Throughout the entire shot sequence, the receiver or receivers are fixed in a borehole drilled in said medium. Subsequently, the receiver or receivers are moved to another position in the borehole for a different shot sequence. This type of acquisition is customarily referred to as "walkaway", that is to say, the shots are made with an offset increasing from one shot to the next.
In borehole seismic surveying on land and/or at sea, use is made of several sources that are separated by an optionally constant distance and are arranged along an acquisition line at the surface of the medium to be explored. A reception tool is lowered into the borehole, the direction of which may be vertical or deviated. The tool generally comprises several receivers which are separated by an optionally constant distance that is generally different than the optionally constant distance separating two consecutive sources. Each receiver comprises three sensors, or geophones, which are oriented in three different directions that are generally orthogonal, one of the directions being specified so as to provide a reference axis. The reference axis may be nonvertical or vertical if these sensors are mounted on a universal joint system. In either case, the reference axis V of the sensors is perpendicular to the plane defined by the other two directions H.sub.1 and H.sub.2, the positions of which are not generally known. The axes V, H.sub.1, H.sub.2 constitute a first system of axes.
The signals representing waves propagating in the medium are recorded as a function of time in the form of traces. The waves propagating through the medium are principally the upgoing or downgoing P-waves (direct waves) and the S-waves.
The traces recorded along the axes V, H.sub.1, H.sub.2 can be grouped as collections of traces with common emitter, for example.
One current way of processing the traces consists in defining a second system of axes R, N, T, in which the axis R is located in the maximum-energy plane. A transformation of this type is performed using what is customarily referred to as a double rotation, one rotation through an angle .PHI..sub.H being performed about the axis V, and the second rotation through an angle .PHI..sub.V being performed about a direction termed H.sub.min, which is perpendicular to a vector H.sub.max that is the projection of the downgoing direct wave into the plane defined by H.sub.1 and H.sub.2, the angle .PHI..sub.H being the angle formed between H.sub.1 and H.sub.max.
The angle .PHI..sub.V is the angle formed by the axes V and R. This transformation of the first system of axes V, H.sub.1, H.sub.2 into a second system of axes R, N, T is carried out, in particular, using software, well-known to those skilled in the art, named SEISLINK which is marketed by the company WESTERN.
It is possible to produce traces in the system of axes R, N, T by projecting the components of the traces identified along the axes V, H.sub.1, H.sub.2, and to form collections of traces with common source identified as a function of the depth z and time t, on the basis of at least the traces along the axis R (or R traces).
The collections of traces, for example V, R or N collections, are then processed to separate the various types of waves, especially the downgoing and upgoing P-waves and the upgoing and downgoing S-waves.
One technique for separating said waves can be implemented by using the SEPAR software from the Compagnie Generale de Geophysique (CGG). However, this requires knowledge of the apparent velocities of the types of waves in question. These apparent velocities are generally determined by manual picking from the recorded traces. The drawbacks of this technique reside in the inherent inaccuracy of manual picking, and sometimes the fact that it is impossible to ascertain the types of waves directly from the recorded traces. Further, the cost of the processing is high while giving not very accurate results.
Another technique, advocated by the company SCHLUMBERGER, consists in using software capable of analyzing the common-source trace collections and of deducing therefrom the apparent velocities of the waves to be separated, when they exist. One drawback of this technique resides in its instability and, above all, in the fact that the separation of the waves is not very efficient because of the small number of sensors used.
These wave separation techniques are described respectively in the following articles:
B. SEEMAN and L. HOROWICZ: "Vertical seismic profiling: Separation of upgoing and downgoing acoustic waves in a stratified medium", GEOPHYSICS, Vol. 48, No. 5 (May 1983), p. 555-568;
C. ESMERSOY: "Inversion of P and SV waves from multicomponent offset vertical seismic profiles", GEOPHYSICS, Vol. 55, No. 1, (January 1990), p. 39-50.